Don't try conversions or extensions on Java companion objects

compiler/src/dotty/tools/dotc/transform/SymUtils.scala

@@ -64,6 +64,8 @@ object SymUtils:
 
     def isSuperAccessor(using Context): Boolean = self.name.is(SuperAccessorName)
 
+    def isNoValue(using Context): Boolean = self.is(Package) || self.isAllOf(JavaModule)
+
     /** Is this a type or term parameter or a term parameter accessor? */
     def isParamOrAccessor(using Context): Boolean =
       self.is(Param) || self.is(ParamAccessor)

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -713,7 +713,7 @@ object Checking {
 
   def checkValue(tree: Tree)(using Context): Unit =
     val sym = tree.tpe.termSymbol
-    if sym.is(Flags.Package) || sym.isAllOf(Flags.JavaModule) && !ctx.isJava then
+    if sym.isNoValue && !ctx.isJava then
       report.error(JavaSymbolIsNotAValue(sym), tree.srcPos)
 
   def checkValue(tree: Tree, proto: Type)(using Context): tree.type =

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -3245,6 +3245,8 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       rememberSearchFailure(qual,
         SearchFailure(qual.withType(NestedFailure(ex.toMessage, selectionProto))))
 
+    if qual.symbol.isNoValue then return EmptyTree
+
     // try an extension method in scope
     try
       val nestedCtx = ctx.fresh.setNewTyperState()

tests/neg/i12537.scala

@@ -0,0 +1,2 @@
+extension [T](x: T) def ext: T = x
+def foo = String.ext  // error

tests/neg/i7750.scala

@@ -1,3 +1,3 @@
-package object A extends runtime.A {
+package object A extends runtime.A {  // error
     implicit def a( : ) = 1() // error // error // error
 }